1. Field of the Invention
The invention relates to high-capacity electrical energy storage. In particular, this invention provides novel electrochemical cells and batteries for large-scale and commercial energy management. More particularly, techniques for delivering and receiving electrical energy by simultaneous ambipolar reversible electrochemical metal extraction in a high-temperature, all-liquid system is described.
2. Background Information
Balancing supply and demand of electrical energy over time and location is a huge unsolved problem in an array of applications all along the course from commercial generator to consumer. The resulting systemic strain reduces the dependability of the supply, causing consumers inconvenience and loss of revenue. Since most electrical energy generation in the United States relies on the combustion of fossil fuels, suboptimal management of electrical energy has a negative environmental impact through excessive emissions of pollutants and greenhouse gases. Moreover, renewable energy sources like wind and solar power are active only intermittently, thus limiting the scale of their deployment in the absence of large-scale storage systems.
Several types of energy storage devices have been proposed to support large-scale and commercial electrical energy management. For example, pumped hydro-storage has been economically used for storing electrical energy for plant load-leveling. Pumped hydro-storage uses a turbine to pump water up a hill. The water is held in a reservoir on the hilltop several hundred feet above a reservoir such as a lake. During the daily high demand period the water is released and the turbine runs in reverse to generate electricity. Although existing hydro-storage systems, mostly built 30 to 40 years ago, operate relatively inexpensively, they are expensive to build. Also, a reservoir sufficient to support an 8-hour discharge translates to a sizeable physical footprint. The technology is inflexible with respect to location, limited to siting on a large, unpopulated flat-topped mountain near an urban setting and close to existing transmission lines.
Electrochemistry-based technologies are more competitive in terms of combining flexibility and compactness with the requisite storage capacity. Conventional lead-acid batteries are the traditional form of large-scale electrochemical energy storage. Highcapacity electricity storage facilities, on the order of 10 MW, have been built using lead-acid cells in vast arrays that are neither compact nor easily relocated. Although they are the cheapest commercial battery technology on the market, their limited cycle life, on the order of several hundred charge-discharge cycles, limits their performance in uses involving frequent activation, such as daily power management. They do not respond well to fast charging or discharging, which lowers their efficiency and reduces their lifespan, as does deep discharge cycling.
Sodium-sulfur (“NAS”) batteries were developed for use in automobiles, but have been adapted to large-scale power management facilities in the US and Japan. The molten sodium and sulfur electrodes sandwich a beta-alumina solid electrolyte. The cell operates at a temperature around 325° C. in a thermal enclosure. The sodium ions pass through the electrolyte to react with sulfur to form a polysulfide. The sulfur is nonconductive and so requires a complex electrode surface to maximize the triple-phase boundary between sulfur, the current collector, and the electrolyte. Thus, the sulfur and polysulfide are held in a carbon-sponge matrix to promote interphase contact.
The technology provides a sufficiently compact apparatus that exhibits an acceptable cycle-life for daily deep-discharges, but there are downsides. The sponge electrode is difficult to manufacture and susceptible to corrosion. The ceramic electrolyte material is fragile and sensitive to thermal shock. Increasing its thickness enhances structural strength but increases the cell's internal resistance, thereby reducing the ion flux and thus the current density during operation. Also, large ceramic bodies of thin cross section are difficult and costly to manufacture. Thus, the design of the cell is constrained by tradeoffs among conductivity, reliability, and ease of manufacturing, so that the cells are not really scalable. Due to the resulting electrolyte size limitations, multiple cells are assembled into long cylinders. An apparatus requires over 20,000 cylinders per MW capacity. Using an array of such a large number of cells complicates integration with, and increases the cost of, the power electronics for interfacing with an AC system.
Despite a longfelt need, a robust energy storage device combining capacity, economy, flexibility and long life has not emerged.